The measurement of the density of a fluid is essential today in many areas, one of the most significant of which is the measurement of the density of fuel in a fuel tank such as a tank incoporated in an aircraft. In addition, it is highly desirable that in addition to a measurement of density an indication of the presence of water in the tank should be obtained. Many devices have been proposed in the past by means of which a measurement of fuel density has been obtained. A common type of such a device utilizes a light-transmitting body of optical material such as glass or the like having various configurations so as to provide reflective and refractive surfaces by means of which light transmitted through the body is altered from its normal path according to the indices of refraction between the optical material of the light-transmitting body and the fluid in which the body is immersed. However, such present-day devices are characterized by many limitations, not the least of which are their complexity and attendant high cost. In addition, such present-day devices for the measurement of density utilize components which must be waterproofed and are provided with surfaces which are adversely affected by bacterial growth in the fluid such as fuel, so that the readings obtained with the device lack the desired degree of accuracy required for proper functioning.